AQA A-Level Biology Paper 1, June 2023: Question 9
11 marks · Hard difficulty · Extended Answer
Evaluate an ELISA test for dengue fever involving monoclonal antibodies, diagnostic accuracy data, and physiological effects on tissue fluid return.
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Question text
09 Dengue fever is a human disease caused by the dengue virus.
Scientists designed an ELISA test to detect antibodies to the dengue virus in a
patient’s blood sample.
Figure 10 shows a diagram of this test and some information about how it works.
Figure 10
Figure 11 shows the negative and positive results that were produced 20 minutes
after the use of the test shown in Figure 10.
Figure 11
09.1 Suggest what is on the test at line T and explain what causes the line to appear in a
positive test.
[2 marks]
09.2 A line at C shows that the test has worked.
Suggest one reason why a line at C shows the test has worked.
[1 mark]
Figure 12 shows a flowchart of how the anti-human antibodies with enzyme attached
are produced.
Figure 12
*0259.*3 Suggest why the fused cells allow continuous production of monoclonal antibodies.
[2 marks]
09.4 Evaluate the ethics of the production process shown in Figure 12.
[1 mark]
Early identification of dengue fever can be difficult as many other diseases produce
the same symptoms. Early identification is important because people suffering with
dengue fever can become ill very quickly and may need hospital treatment.
Scientists compared the effectiveness of three diagnostic tests for dengue fever.
• Laboratory-based test – a patient’s blood sample is sent from the doctor’s clinic to a
laboratory for testing.
• Current test used in the doctor’s clinic.
• New test to be used in the doctor’s clinic – the ELISA test shown in Figures 10 and
11 (on page 24).
The scientists’ results are shown in Table 3.
A blood sample from each patient with confirmed dengue fever at each time after
onset of symptoms was tested with all three diagnostic tests.
Table 3
Number of Number of positive results
Time after onset
confirmed
of symptoms /
dengue fever Laboratory-
days Current test New test
patients tested based test
1–2 14 10 0 6
3–4 38 28 6 24
5–7 18 29 8 14 14
09.5 The scientists recommend that the new test is used for the identification of dengue
fever in all countries around the world.
Discuss this recommendation. Use all the information given.
[3 marks]
*0289.*6 The dengue virus causes damage to capillaries so that blood proteins move out of the
capillaries into the tissue fluid.
Explain how this would affect the return of tissue fluid into the capillaries.
[2 marks]
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
1. Antigen (at T and substrate); 1. Reject antigen in
blood
2. Enzyme-substrate complex (produces a
line/colour change) 2. Accept colourless
09.1 (2 x dye for substrate
OR
AO2)
2. Accept ES complex
Enzyme (binds) with substrate (produces
in this instance
line/colour change);
Accept Blood/sample
Blood/sample has moved/diffused (above T in the 1
09.2 and anti-human
test);
(AO3) antibody have moved
(in the test)
1. Accept mitosis OR
1. (Cancer/fused cells) divide/replicate
reproduce for divide
rapidly/uncontrollably;
1. Accept hybridoma
2. B cells produce (monoclonal) antibody;
2 for ‘fused cell’
09.3 (2 x 2. Accept ‘plasma
AO2) cells’ OR ‘memory
cells’ for ‘B cells’
2. Accept secrete OR
make for produce
Harmful but not killed Must have idea ‘for’
OR and idea ‘against’
Harmful but only used once Accept stressed OR
exploited OR
OR
1 mistreated OR
Harmful but stops human suffering
09.4 abused for harmed
OR (AO3)
Accept illness OR
Harmful but produces (useful) medicine/drugs infection OR death for
OR suffering
Not harmed but injected (with a substance);
Max 2 from 4, 5 and 6
Accept ELISA test for
new test
1, 2 and 3 Accept 23
correct comparative
1. Better than current at detecting early but not as figures from Table 3
good as lab-based; OR calculations
(some examples in
2. New/current test better than lab-based from 5 table below)
days
1. and 2. Accept
OR higher proportion for
New test as good as current from 5 days; better
3. New test and lab-based better total (of) positives 1. Accept reference to
than current day(s) in range of 1
to 4 days for early
OR 3 max
1. Accept identifies
09.5 New test total (of) positives not (quite) as good (3 x
OR finds OR spots
as lab-based; AO3) for detects
4. New test (likely to be) quicker; 1. and 2. Accept
5. New test (likely to be) cheaper; ‘more accurate’ OR
‘identifies more’ OR
6. Limited/inaccessible labs ‘finds more’ OR
OR ‘detects more’ OR
‘is more successful’
Limited training of people to use labs; OR ‘is more
effective’ for “better
than” and the
converse of these
statements for “not
as good as”
2. Accept after 4 days
4. Accept ‘more
efficient’ for quicker
Time after onset of
Number of positive results / % (combined with earlier result(s))
symptoms / days
Lab-based test Current test New test
1–2 71 0 43
3–4 74 (73) 16 (12) 63 (58)
5–7 44 (66) 78 (29) 78 (63)
1. Increases water potential of blood/capillary 1. Accept Ψ for water
potential
OR
1 Ignore WP
24 Decreases water potential of tissue fluid; 2 1. Accept reduces
09.6 water potential
2. (So) less water returns to blood/capillaries (by (2 x
gradient
osmosis) AO2)
2. Ignore tissue fluid
OR
2. Accept ‘no’ for less
(So) more water leaves blood/capillaries (by
osmosis);
How to answer it
Dengue Virus ELISA Test & Tissue Fluid Study Guide
What this question tests
This exam question evaluates your understanding of immunology, specifically ELISA tests, monoclonal antibody production via hybridomas, ethical considerations in animal testing, data interpretation and evaluation of diagnostic tests, and mass transport/water potential principles in tissue fluid exchange.
ELISA Test Mechanism at Line T
Suggest what is on the test at line T and explain what causes the line to appear in a positive test. [2 marks]
✅ Correct Answer
- 1. Dengue antigen (and a substrate for the enzyme).
- 2. Enzyme-substrate complex forms, producing a line/colour change (or enzyme binds with substrate to produce a colour change).
💡 Key Knowledge
In this ELISA setup, anti-human antibodies with an attached enzyme flow down the strip. If the patient has dengue antibodies in their blood sample, these antibodies will bind to the antigen fixed at test line T. The enzyme attached to the secondary antibody then acts on a colourless substrate to yield a visible coloured line.
❌ Common Errors
Students frequently confuse antigens with antibodies or state that "antibodies bind to antibodies" at line T without specifying the antigen component. Do not state that antigen is in the blood (reject antigen in blood).
Control Line Function
Suggest one reason why a line at C shows the test has worked. [1 mark]
✅ Correct Answer
The blood, sample, or anti-human antibodies have successfully moved/diffused up and across the test strip.
🧠 Exam Technique
Keep your answer concise. The control line confirms that capillary action / flow of liquid has occurred properly across the entirety of the testing membrane.
Monoclonal Antibody Production
Suggest why the fused cells allow continuous production of monoclonal antibodies. [2 marks]
✅ Correct Answer
- 1. Cancer cells divide/replicate rapidly and uncontrollably (mitosis).
- 2. B cells produce the specific (monoclonal) antibody.
💡 Key Knowledge
This process describes creating a hybridoma. Normal antibody-producing B cells (plasma cells) do not divide indefinitely in culture. Cancer (myeloma) cells divide continuously. Fusing them combines immortality with antibody-secreting capability.
❌ Common Errors
Vague references to "cells multiplying" without linking cancer cells to rapid division and B cells to antibody production will lose marks. Ensure both cell types' functions are clearly distinguished.
Ethics of Animal Testing
Evaluate the ethics of the production process shown in Figure 12. [1 mark]
✅ Correct Answer
Harmful to the animal (e.g. stressed, exploited, mistreated, injected with a substance) but not killed / produces useful medicine / stops human suffering.
🧠 Exam Technique
For a 1-mark evaluation question involving animal models, examiners look for balance or a clear "for and against" justification (e.g. acknowledging animal harm vs. medical benefit).
Evaluating Diagnostic Test Data
The scientists recommend that the new test is used for the identification of dengue fever in all countries around the world. Discuss this recommendation. Use all the information given. [3 marks]
✅ Correct Answer (Max 3 from points below):
- New test is better than the current test at detecting early cases (days 1-2), but not as good as the lab-based test.
- New/current test is better than lab-based from 5-7 days (or new test is as good as current test from 5-7 days).
- New test and lab-based tests have a higher total number of positives than the current test.
- New test is likely to be quicker or cheaper than lab-based testing.
- Limitations: Inaccessible labs or lack of training in certain countries make a doctor's clinic test advantageous.
🧠 Exam Technique
Always use comparative statements and quote figures or calculate totals from Table 3 to back up your points. Avoid saying a test is simply "good" or "bad"—compare New vs Lab-based vs Current across different time windows (1-2 days, 3-4 days, 5-7 days).
Tissue Fluid Return
The dengue virus causes damage to capillaries so that blood proteins move out of the capillaries into the tissue fluid. Explain how this would affect the return of tissue fluid into the capillaries. [2 marks]
✅ Correct Answer
- 1. Increases water potential of blood / capillaries (or decreases water potential of tissue fluid / reduces water potential gradient).
- 2. Therefore, less water returns to the blood/capillaries by osmosis (or more water leaves blood/capillaries).
💡 Key Knowledge
Under normal hydrostatic and oncotic pressure conditions, blood proteins (plasma proteins) lower the water potential of the blood plasma, drawing water back into the capillaries at the venule end via osmosis. If proteins leak into the tissue fluid, the water potential gradient is diminished or reversed, leading to fluid accumulation (oedema).
❌ Common Errors
Do not confuse blood water potential with blood pressure. Examiners penalise answers that refer to "pressure gradients" when the question specifically hinges on the movement of proteins altering water potential.
Topics
Biology · 3.2 Cells · 3.3 Organisms exchange substances with their environment
Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.